ui: Clarify help messages for 'message' fields

Add or amend tooltips to clarify what the message is for and that it
doesn't go over the network.
Merge pull request #3571

57feb1b ui: Clarify help messages for 'message' fields (Wladimir J. van der Laan)
Unit test fixes for AssertLockHeld / -DDEBUG_LOCKORDER

Unit tests would fail if compiled with -DDEBUG_LOCKORDER (AssertLockHeld()
would fail; AssertLockHeld() relies on the DEBUG_LOCKORDER code to keep
track of locks held).

Fixed by LOCK'ing the wallet mutex in the unit tests that manipulate the
wallet.